This application claims Paris Convention priority of DE 102 02 372.7 filed Jan. 23, 2002 the complete disclosure of which is hereby incorporated by reference.
The Invention concerns an NMR (nuclear magnetic resonance) high field magnet coil system comprising superconducting conductor structures with several solenoid-shaped radially nested coil sections for generating a homogeneous magnetic field B0 in a measuring volume.
An arrangement of this type is known e.g. from the manual of the TU Munich xe2x80x9cSupraleitende Magnete fxc3xcr die NMR-Spektroskopiexe2x80x9d, http://www.org.chemie.tu-muenchen.de/people/rh/nmrueb/magnet2.pdf (status: 21.12.2001) by Rainer Haexcex2ner.
An alternative construction of an NMR high field magnet coil arrangement in pancake arrangement is disclosed in Martin N. Wilson, Superconducting Magnets, Oxford University Press, New York, 1983/1997, page 318.
Nuclear magnetic resonance (NMR) spectroscopy is currently of great importance for structural examination in chemistry and solid-state physics and also in medical diagnostics for measurement of the spatial density distribution of certain atoms, e.g. protons in living tissue to supplement and extend X-ray diagnostics (Nuclear Magnetic Resonance Tomography or Magnetic Resonance Imaging).
In NMR, maximum magnetic field strengths are usually desired in a measuring volume with simultaneous maximum homogeneity and temporal stability. All these factors decisively contribute to the resolution capability of the NMR measurement.
High magnetic field strengths can be obtained through use of superconducting magnet coil systems. Electrical currents of up to a few hundred amperes, which produce magnetic fields on the order of a few Tesia, flow with almost no loss in the superconducting coils which are short-circuited during operation via a superconducting bridge.
The homogeneity and stability of the magnetic fields is obtained through additional external compensation coils, so-called shim coils, and also through as symmetrical a structure of the magnet arrangement as possible. The shim coils can also be designed as superconductors.
When superconductors are used, one must take into account the fact that superconductors undergo a transition into the normally conducting state above certain temperatures, electrical currents and magnetic field strengths which depend on the material used. The critical maximum of any parameter thereby depends on the size of the two other parameters.
Since 1986, so-called high-temperature superconductors (HTS) are available which have higher transition temperatures and higher critical magnetic field strengths than conventional (in particular metallic) superconductors. HTS materials are therefore preferably used for the inner coil sections of magnet coil systems, since the largest magnetic field strengths act at that location.
HTS materials generally have ceramic properties and are therefore difficult to process due to low ductility and low breaking strength. The HTS must usually be produced in the shape in which it is used. In particular, strong bends (kinks) in the superconducting structure are problematic and can considerably reduce the current carrying capability of the superconductor. Minimum possible radii of curvature are therefore given for these brittle superconductors which must be taken into consideration for the application at hand.
Another problem arises when the superconductors are to be electrically connected, in particular when connecting two partial superconducting sections of a conductor arrangement. A superconducting solder is used at the contacting points, the so-called joints, which however usually has a lower critical magnetic field strength than the other superconducting material. The fields of application of the arrangement is limited by the solder unless the joints can be disposed in a region of lower magnetic field strength than the main part of the superconducting arrangement.
The Pancake magnetic coil arrangement described by Wilson loc. cit., which is based on a tape winding technology, leaves the joints in the high field region and the solder limits the maximum achievable magnetic field strength such that use of HTS materials in this arrangement is not reasonable.
In the arrangement of MAGNEX as described by Haexcex2ner, loc. cit., the joints of the radially nested solenoidal coil sections are drawn into the low field region. However, in the geometry shown, the superconductor must be extracted parallel to the winding axis, i.e. perpendicular to the winding direction. This requires a bend of approximately 90xc2x0 in the conductor path of the superconductor as a result of which, the required minimum admissible bending radius of the coil material is not determined by the coil radius but by the bend in the conductor path. For this reason, ceramic HTS materials cannot be used in this arrangement, since their minimum bending radius is excessively large.
In contrast thereto, it is the object of the present invention to provide for use of a brittle, band-shaped superconductor in the innermost coil system which is incapable of being bent to such a large extent at the upper edge of the system.
In accordance with the invention, this object is achieved in a surprisingly simple and effective fashion in that the radially innermost coil section is wound with a band-shaped superconductor, having an aspect ratio (width to thickness) greater than 3, onto a coil support which, at least at one axial end, projects in an axial direction past the winding packet of the radially neighboring coil section, wherein the band-shaped superconductor is tangentially, outwardly guided at that end into a region of reduced magnetic field strength to terminate in at least one electrical connecting point.
The superconductor can therefore be guided from the high field region of the magnet arrangement without substantial bend and contacting of the superconductor can be effected by the solder without impairing the maximum magnetic field strength. The magnet arrangement with radially nested solenoidal coil sections is therefore also easy to realize using brittle ceramic superconductors, which include the known HTS materials.
In a particularly advantageous embodiment of the inventive magnet coil system, the band-shaped superconductor contains high-temperature superconducting (HTS) material. This produces considerably higher magnetic field strengths and the resolution of an associated NMR measuring means can be considerably increased.
In one particularly advantageous embodiment, the at least one electrical connecting point in which the tangentially outwardly directed band-shaped superconductor terminates, is superconducting. This connecting point is subjected to a lower magnetic field strength than the main part of the coil such that the superconducting solder which is usually used does not limit the applications of the arrangement, in particular the maximum magnetic field strength.
In a preferred further development of this embodiment, the electrical connecting point is a superconducting switch to facilitate charging of the magnet coil arrangement.
In one further particularly preferred embodiment of the magnet arrangement, at least one of the electrical connecting points connects different superconducting materials to permit change of the superconducting material within the circuit, in particular, to an easily handled, ductile metallic superconductor.
In one additional advantageous embodiment, the band-shaped superconductor is returned from the electric connecting point to the magnet coil thereby permitting more winding layers or larger winding lengths and therefore higher magnetic field strengths.
In one advantageous embodiment of the magnet arrangement, the other axial end of the coil support of the radially innermost coil section does not project axially past the winding packets of the remaining coil sections of the magnet coil system. This reduces the overall height of the system and saves material.
In one particularly preferred embodiment of the inventive magnet arrangement, the radially innermost coil section is wound in the axial direction in an asymmetric manner relative to the magnetic center of the outer coil sections in such a fashion as to maintain the homogeneity of the overall magnetic field required for an NMR high field magnet coil system. This obviates the need for further additional correction coils for field homogenization.
In one particularly advantageous embodiment, the winding layers of the radially innermost coil section having the band-shaped superconductor tangentially guided at one axial end outwardly towards a region of reduced magnetic field strength, have an axially varying winding density. The magnet coil arrangement in accordance with prior art with outer coil sections each projecting past the neighboring inner coil sections (see Haexcex2ner, loc. cit.) has the advantage of high magnetic field homogeneity in the center of the arrangement. This geometry is lost by the inventive protrusion of the innermost coil section. The inventive variation of the winding density, in particular a reduction in the winding number in the projecting region, can keep the resulting disturbance low.
In a further advantageous embodiment of the inventive magnetic field arrangement, the band-shaped superconductor is precisely guided through specially formed spacers having the same height as the band-shaped superconductor. After a layer is completely wound, this produces a uniform cylindrical surface as a basis for further possible windings.
In another advantageous embodiment, the parts of the band-shaped superconductor which are tangentially outwardly guided into a region of reduced magnetic field strength and optionally inwardly, tangentially returned from that location are guided through groove-like depressions on the surfaces of the coil support of the radially innermost coil section. This fixes the position of the superconductor such that the forces on the conductor (i.e. the Lorenz forces in the magnetic field) cause no change in the conductor geometry which would, in turn, cause a magnetic field change.
In an advantageous further development of this embodiment, the groove-like depressions are directly integrated in the coil support for defined guidance of the band-shaped superconductor of the radially innermost winding layer in a particularly simple and mechanically highly stable fashion.
In another further development, flexible mats with depressions for defined guidance of the neighboring winding layer(s) of the band-shaped superconductor are disposed between the winding layers of the band-shaped superconductor of the radially innermost coil section to permit mechanical fixing of several winding layers.
In an alternative further development, two folded together partial shells with depressions for defined guidance of the neighboring winding layer(s) of the band-shaped superconductor are disposed between each winding layer of the band-shaped superconductor of the radially innermost coil section to also permit mechanical fixing of several winding layers.
In one further particularly advantageous development, the groove-like depressions in the surfaces of the coil support or in the underlying mats or partial shells have saw-toothed cross-sections. The band-shaped superconductor can thereby be held in the guidance when subjected to forces and without changing shape, despite the pitch of the coil winding along the coil axis. The applied forces are directed largely normal to the surface of the band to keep mechanical loading of the band at a minimum.
In another advantageous embodiment of the inventive magnet arrangement, the cross-section of the coil support of the radially innermost coil section extends in an axially variable manner, in particular conically. This also contributes to maintaining the homogeneity of the magnetic field close to the center which is disturbed by the projecting innermost coil section.
In a further advantageous embodiment, the electrical connecting points are uniformly and azimuthally distributed thereby rendering the disturbances of the magnetic field caused by the connecting points symmetrical to largely maintain the homogeneity of the magnetic field.
In another advantageous embodiment, the electrical connecting points are azimuthally adjacent. Magnetic field disturbances of the connecting points are thereby limited to a confined spatial region.
In a particularly advantageous embodiment of the inventive magnet arrangement, the minimum admissible bending radius of the band-shaped superconductor is of the same order of magnitude as the radius of the coil support of the radially innermost coil section. The coil geometry is thereby optimally adjusted to the brittle superconducting material used such that the advantages of the invention, in particular the use of brittle HTS materials in the inventive coil geometry, is optimally utilized for generating maximum magnetic field strengths.
This is particularly significant in one advantageous embodiment with which the magnet coil system is designed to produce a magnetic induction strength greater than 20 T to further improve the resolution in NMR measurements.
In a further advantageous embodiment of the inventive magnet arrangement, the operating temperature is approximately equal to or smaller than 4 K, in particular approximately 2 K. This low operating temperature permits use of high magnetic field strengths so that the advantage of the inventive geometry which, i.a. displaces the joints to a region of a low magnetic field strength, is fully utilized.
In a further embodiment of the inventive magnet arrangement, all coil sections are connected in series. This connection is particularly simple.
In another embodiment, all sections are in a constant current mode during operation.
In a further advantageous embodiment of the inventive magnet arrangement, all sections having no HTS are connected in series to electrically combine one part of the coil arrangement in a reasonable and simple fashion.
In one further development, a device is provided to operate the section containing the HTS with a separate power supply. This power supply can be matched to the requirements of this particular section.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be used in accordance with the invention individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for illustrating the invention.
The invention is shown in the drawing and explained in more detail by means of embodiments.